1. Field of the Invention
The present invention relates to a novel chemoattractant receptor and its use. More particularly, this invention relates to a novel heptahelix-type chemokine receptor which naturally binds leukotriene B4 and also serves as a coreceptor facilitating entry of human immunodeficiency virus type 1 (HIV-1) into cells expressing CD4, and methods for using this novel receptor in HIV infectivity and drug screening studies.
2. Description of Related Art
It is now well understood that HIV-1 infection is initiated by interaction of the virion envelope glycoproteins (gpl120/41) with at least two classes of cell membrane receptors. First, the virus associates with the CD4 receptor (1-3), which induces conformational changes in the glycoprotein envelope (4, 5), allowing the virus to subsequently bind to a seven-transmembrane envelope with the cell membrane leading to viral entry. Two major coreceptors have been identified, both belonging to the chemokine family of G-protein coupled receptors.
Macrophage-tropic (M-tropic) strains of HIV, which replicate in macrophages and CD4+T-cells use the .beta.-chemokine receptor CCR5 (6-10). T-tropic isolates of HIV, which replicate in primary CD4+T cells, established CD4+T cell lines, as well as macrophages, use the .alpha.-chemokine receptor CXCR4 (11).
A key observation leading to the recent discovery of the viral entry cofactors was the finding that certain .beta.-chemokines have a strong suppressive effect on the HIV-1 infection in vitro (12). Analysis of viral isolates obtained sequentially from infected individuals has shown a loss of sensitivity to inhibition by .beta.-chemokines along with a shift in virus phenotype from a non-syncytium-inducing (NSI) to a syncytium-inducing (SI) phenotype (13-17). This suggests that there is a shift in chemokine receptor usage from CCR5 to CXCR4 as the infection progresses. This is in accordance with the findings that dual-tropic virus strains utilize both types of coreceptors, and that additional coreceptors exist for certain subsets of primary viruses, in addition to their primary usage of CCR5 or CXCR4 (9, 10, 18-20). Hence, the usage of the two major cofactors may be viewed as extremes in an adaption process, along which the virus expands its coreceptor usage to include several different receptors.
Among the superfamily of G-protein coupled, or heptahelix, receptors the chemokine ones form a structurally related group that belongs to the subfamily of leukocyte chemoattractant receptors, which also includes receptors for the so-called classical chemoattractants (24).
Recently, a novel chemoattractant-like receptor, CMKRL1, was cloned (25) whose natural ligand subsequently was shown to be leukotriene B4 (26, 27). This is the first cloned leukotriene receptor, although the leukotrienes themselves, formed from arachidonic acid through the lipooxygenase pathway, have been known for more than two decades (28). The leukotriene B4 receptor, or BLTR, is widely expressed in the immune system (25, 27)--including thymus, spleen, lymph nodes and PBMC--and it shows approximately 30% identity with CCR5 and CXCR4 which, in turn, exhibit the same degree of homology when compared to each other.
Because of the continuing interest in understanding the biology of HIV infection and ways to block such infections, there exists a need in the art for identifying additional HIV coreceptors. In particular, there exists a need in the art for information on the identity, characterization, and efficacy of additional HIV coreceptors. The identification and characterization of additional coreceptors would be particularly advantageous in developing specific therapies for preventing primary HIV infection and limiting the spread of such infections once they have occurred.
Therefore, it is important to search for additional G-protein-coupled receptors that might facilitate the entry of various primary HIV-1 isolates into suitable target cells during the course of the infection. Further, it is important to identify pharmacologically active compounds that inhibit the interaction between the initial HIV-CD4 complex and the coreceptor.